The present invention relates to the configuration of field devices of process control and monitoring systems. In particular, the present invention relates to configuration of field devices in a digital communication network.
In a typical industrial plant, a process control/monitoring system is used to control many of the industrial processes performed at the plant. Typically, the plant has a centralized control room having a computer system with user input/output (I/O), disc I/O, and other peripherals as are known in the computing art. Coupled to the computing system are a controller and a process I/O subsystem.
The process I/O subsystem includes I/O ports which are connected to various field devices throughout the plant. Field devices include various types of analytical equipment, silicon pressure sensors, capacitive pressure sensors, resistive temperature detectors, thermocouples, strain gauges, limit switches, on/off switches, flow transmitters, pressure transmitters, capacitance level switches, weigh scales, transducers, valve positioners, valve controllers, actuators, solenoids, and indicator lights. The term “field device” encompasses these devices, as well as any other device that performs a function in a process control/monitoring system.
Traditionally, analog field devices have been connected to the control room by two-wire twisted pair current loops, with each device connected to the control room by a single two-wire twisted pair. Analog field devices are capable of responding to or transmitting an electrical signal within a specified range. In a typical system, it is common to have a voltage differential of approximately 20-25 volts between the two wires of the pair and a current of 4-20 milliamps running through the loop. An analog field device that transmits a signal to the control room modulates the current running through the current loop, with the current proportional to the sensed process variable. On the other hand, an analog field device that performs an action under control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the I/O port of the process I/O system, which in turn is controlled by the controller.
More recently, hybrid systems that superimpose digital data on the current loop have been used in process control systems. One hybrid system known in the control art as the Highway Addressable Remote Transducer (HART) system uses the magnitude of the current in the current loop to sense a process variable (as in the traditional system), but also superimposes a digital carrier signal upon the current loop signal. The carrier signal is relatively slow, and can provide updates of a secondary process variable at a rate of approximately 2-3 updates per second. Generally, the digital carrier signal is used to send secondary and diagnostic information and is not used to realize the primary control function of the field device. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (including sensor diagnostics, device diagnostics, wiring diagnostics, and process diagnostics), operating temperatures, temperature of the sensor, calibration information, device ID numbers, materials of construction, configuration or programming information, etc. Accordingly, a single hybrid field device may have a variety of input and output variables and may implement a variety of functions.
Fieldbus is a multi-drop serial digital two-way communications protocol defined by the Instrument Society of America (ISA). Fieldbus is intended for connecting field instruments and other process devices such as monitoring and simulation units in distributed control systems. Fieldbus allows enhanced digital communication over previous process control loop methods while maintaining the ability to power process devices coupled to the Fieldbus loop and while meeting intrinsic safety requirements.
Two reasonably standardized industrial Fieldbus protocols are Foundation Fieldbus and Profibus. The physical layer of the Fieldbus protocols are defined by Instrument Society of America standard ISA-S50.02-1992, and its draft two extension dated 1995. The Fieldbus protocol defines two subprotocols. An H1 Fieldbus network transmits data at a rate up to 31.25 kilobits per second (Kbps) and provides power to field devices coupled to the network. The H1 physical layer subprotocol is defined in Clause 11 of the ISA standard, part two approved in September 1992. An H2 Fieldbus network transmits data at a rate up to 2.5 megabits per second (Mbps), does not provide power to field devices connected to the network, and is provided with redundant transmission medium.
Foundation Fieldbus allows each field device in the network to be configured. Configuration data is stored in nonvolatile memory within the field device, and is used to define the function characteristics of the field device on the network. This configurability of field devices allows a Foundation Fieldbus network to be used in a wide array of different applications. On the other hand, configuration of a Foundation Fieldbus field device can be complex, and requires substantial skill and experience by the individual performing the configuration. Often, this requires that the person configuring the field device be a control systems engineer.
In addition, maintenance can be more complex because it is not possible to simply remove one field device from the Foundation Fieldbus network and replace it with another field device. Instead, field devices must be recommissioned as well as reconfigured every time a field device is replaced. Replacing a Foundation Fieldbus field device requires operation of the host computer, which includes a configuration tool in software. The host stores data relating to the configuration of the device to be replaced in its database. The device is then decommissioned and replaced with a new device. The new device is then commissioned and is configured with the stored configuration data from the host. The user must then run device specific methods at the host to configure miscellaneous features of the field device that has been installed.
Configuration by the host involves sending a sequence of individual messages to each field device in order to configure every function block and every transducer block. There may be, for example, up to 5 transducer blocks and 13 function blocks requiring configuration. This configuration process must be performed from the host (which has the configuration tool software), rather than at the location where the field device is installed.